1. Technical Field
The present invention relates to an electric field enhancement element and a Raman spectrometer.
2. Related Art
Recently, as a highly sensitive spectroscopic technique for detecting a sample molecule at a low concentration, an affinity sensor utilizing localized surface plasmon resonance (LSPR) or surface-enhanced Raman scattering (SERS) for qualitative and quantitative detection directly from vibrational spectroscopy has attracted attention. SERS is spectroscopy in which an enhanced electric field is formed on a metal surface with a nanometer-scale rough structures, and Raman scattered light is enhanced by 102 times to 104 times, thereby enabling detection with high sensitivity. A target molecule (target substance) is irradiated with linearly polarized excitation light with a single wavelength such as a laser, and scattered light (Raman scattered light) with a wavelength which is shifted from the wavelength of the excitation light by the molecular vibration energy of the target molecule is spectroscopically detected, whereby a fingerprint spectrum is obtained. Based on the shape of this fingerprint spectrum, the target substance can be identified.
It has been confirmed that there is a correlation between tracheal inflammation due to asthma and the concentration of NO (nitrogen monoxide) contained in the breath, and the concentration of NO in the breath has been recognized as an indicator of asthma. As a sensor chip for detecting such NO, a sensor chip focusing on surface plasmon resonance (SPR) which is a light sensing technique has been proposed. For example, APPLIED SPECTROSCOPY, Volume 65, Number 8, 825-837, 2011 (Non-Patent Document 1) describes that a biological enzyme called “cytochrome P450” is disposed on an LSPR substrate having an Ag microstructure, and NO is allowed to react with the cytochrome P450 enzyme, and the SERS signal of the reactant is obtained.
Here, the distribution of the enhanced electric field induced by surface plasmon resonance exponentially decreases as the distance from the surface of the metal nanoparticle (metal microstructure) of the sensor chip (electric field enhancement element) increases. For example, the result of a test calculation shows that the degree of enhancement at a place more than 3 nm away from the surface of the metal microstructure is decreased to 1/200 or less of the degree of enhancement on the surface of the metal microstructure. Therefore, it is considered that the sensitivity of sensing using surface plasmon resonance increases as the target molecule is located closer to the surface of the metal microstructure.
The cytochrome P450 described in Non-Patent Document 1 is composed of about 500 amino acid residues and is a high molecular weight protein with heme in the active site. Due to this, even if the cytochrome P450 is disposed on a metal microstructure which induces plasmon resonance, it is difficult to dispose an NO binding portion (binding site) at a distance of 3 nm or less from the surface of the metal microstructure. Therefore, NO may sometimes not be able to be detected with high sensitivity by the technique described in Non-Patent Document 1.